The multipath reception of NTSC television images, commonly referred to ghosting, is a widespread problem both for signals received directly off-air and for signals received from cable TV systems. Recent advances in digital signal processing technology make it both practical and economical to implement a ghost canceling system is consumer television receivers that will eliminate, or at least substantially reduce, the deleterious effects caused by multipath reception.
Ghost images, commonly referred to a "ghosts", are as common occurrence in received television pictures. As compared with the predominant image produced by a signal received by a direct path, a ghost is produced by a time-delayed, usually attenuated, and distorted version of the proper television signal, having been received by way of a path other than the direct path. A signal path other than the direct path is referred to as a multipath. Generally, the delay and attenuation may be either positive or negative. Thus, it is possible to have a pre-ghost or a ghost with a larger amplitude than the main signal. The parameters of a ghost signal may also be time-varying.
Multipaths can be broadly classified into long and short multipaths. A long multipath manifests itself as a secondary ghost image displaced horizontally with respect to a predominant image, whereas a short multipath influences high video frequencies. Its effect is typically observable as an apparent increase or decrease in image sharpness, accompanied in some cases by loss of some image information. The attenuation of high video frequencies can lead to a `soft` appearance in the picture. Short multipaths are typically of concern in cable distribution systems and generally arise from termination mismatches and multiple echoes, commonly called "macro-ghosts". Long multipath ghosts are typically reduced by cancellation schemes whereas short multipath effects are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the high frequency video response.
The phenomenon of television ghosts has been addressed with a view to improving picture quality by reducing or eliminating ghosts. See, for example, W. Ciciora et al., A Tutorial On Ghost Canceling In Television Receivers, published in the IEEE Transactions On Consumer Electronics, volume CE-25, during February 1979, at pages 9-43. Other solutions to the problem of ghosts are described in U.S. Pat. No. 4,896,213, Jan. 23, 1990 to Kobo et al. and U.S. Pat. No. 4,897,725, Jan. 30, 1990, to Tanaka et al., the disclosures of which patents are herein incorporated by reference.
Since the characteristics of a transmitted television signal are known a priori, it is possible, at least in theory, to utilize such characteristics in a system of ghost signal detection and cancellation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of the television signal that is currently unused for video purposes and to use this reference signal for detection and cancellation of ghost signals. Typically, lines in the vertical blanking interval (VBI) are used. Such a signal is herein referred to as a Ghost Canceling Reference (i.e., a "GCR") signal.
It has been proposed that a useful test or GCR signal may appropriately exhibit a (sin x)/x waveform. Such a waveform, suitably windowed, exhibits a relatively constant spectral energy density over a frequency band of interest. See, for example, A Tutorial On Ghost Canceling In Television Receivers by W. Ciciora et al. Ghost locations can then be determined for ghost signal cancellation and waveform equalization to reduce the effects of short multipaths.
U.S. Pat. No. 4,896,213 to Kubo notes a ghost canceling signal transmission/reception system which allows a built-in ghost canceling device to reduce or eliminate ghost components attributable to group-delay distortion and frequency-amplitude characteristic distortion generated in a signal transmission path. This is achieved by superimposing a digital signal on a television signal as a ghost canceling reference signal. Thus, in U.S. Pat. No. 4,896,213, a digital signal composed of frame synchronizing signals, clock synchronizing signals, and data signals is generated, and is superposed, during the vertical blanking interval, on a television signal to be transmitted. At the receiving end, the digital signal superposed on the television signal is used as a reference signal in an arrangement that executes a correlative operation of the transmitted television signal to reduce the ghost phenomenon.
In the arrangement of U.S. Pat. No. 4,897,725 to Tanaka, a transmitted reference or GCR signal is also used. A dummy ghost signal is generated and is used for canceling a ghost signal in the transmitted television signal. This is substantially the proposed BTA (Japan) GCR signal, which uses as the main reference, or deghosting, signal a signal having the aforementioned (sin x)/x waveform, principally for its property of a substantial high frequency spectral energy content. Averaging with a pair-wise constant signal is used for deriving a received reference waveform. The received reference waveform is Fourier transformed to provide a set of Fourier coefficients. The transformed reference waveform is then processed with an available Fast Fourier Tranform of an unimpaired GCR to compute the deghosting filter parameters, that is, tap gain information for a transversal filter, for both waveform equalization finite impulse response (i.e., "FIR") and the deghosting filter infinite impulse response (i.e., an "IIR" filter).
As can be expected, the ghost cancelation reference signal is generally received accompanied by its ghost signals and is thus itself a "ghosted" signal. It is herein recognized that the performance of a ghost-canceling system is greatly influenced by the noise and perturbation content of the acquired GCR signal. It is also recognized that a reduction in the noise and perturbation content of the acquired GCR signal is desirable in improving the accuracy of the deghosting filter parameter derivations and in reducing the system complexity.
It is herein further recognized that a step in the signal leading edge is desirable in a GCR signal in computing ghost locations. As previously mentioned, a (sin x)/x waveform provides particular advantages in a GCR signal. Its flat frequency spectrum allows accurate computation of the filter parameters for attenuating multiple image effects as well as computation of the waveform equalizing parameters. The characteristic ripples of the (sin x)/x waveform however, along with other high frequency components, are typically attenuated in a received ghosted GCR, both due to multipath effects as well as effects of antenna misorientation as commonly occurs in practice. Under such conditions, the computation of the waveform equalizing parameters can be significantly in error.